OX40 (CD134; TNFRSF4) is a tumor necrosis factor receptor found primarily on activated CD4+ and CD8+ T-cells, regulatory T cells (Treg) and natural killer (NK) cells (Croft et al., 2009, Immunol Rev. 229:173-91). OX40 has one known endogenous ligand, OX40 ligand (OX40L; CD152; TNFSF4), that exists in a trimeric form and can cluster OX40 resulting in potent cell signaling events within T cells (Croft et al., 2009, Immunol Rev. 229:173-91). Signaling through OX40 on activated CD4+ and CD8+ T cells leads to enhanced cytokine production, granzyme and perforin release and expansion of effector and memory T cell pools (Jensen et al., 2010, Semin Oncol. 37:524-32). In addition, OX40 signaling on Treg cells inhibits expansion of Tregs, shuts down the induction of Tregs and blocks Treg-suppressive function (Voo et al., 2013, J Immunol. 191:3641-50; Vu et al., 2007, Blood. 110:2501-10).
Immunohistochemistry studies and early flow cytometry analyses showed that OX40 is expressed on T cells infiltrating a broad range of human cancers (Baruah et al., 2011, Immunobiology 217:668-675; Curti et al, 2013, Cancer Res. 73:7189-98; Ladanyi et al, 2004, Clin Cancer Res. 10:521-30; Petty et al, 2002, Am J Surg. 183:512-8; Ramstad et al, 2000, Am J Surg. 179:400-6; Sarff et al, 2008, Am J Surg. 195:621-5; discussion 625; Vetto et al, 1997, Am J Surg. 174:258-65). OX40 expression on tumor-infiltrating lymphocytes correlates with longer survival in several human cancers, suggesting that OX40 signals may play a critical role in establishing an antitumor immune response (Ladanyi et al., 2004, Clin Cancer Res. 10:521-30; Petty et al., 2002, Am J Surg. 183:512-8).
In a variety of nonclinical mouse tumor models, agonists of OX40, including antibodies and OX40 ligand fusion proteins, have been used successfully with promising results (Kjaergaard et al., 2000, Cancer Res. 60:5514-21; Ndhlovu et al., 2001, J Immunol. 167:2991-9; Weinberg et al., 2000, J Immunol. 164:2160-9). Co-stimulating T cells through OX40 promoted anti-tumor activity that in some cases was durable, providing long-lasting protection against subsequent tumor challenge (Weinberg et al., 2000, J Immunol. 164:2160-9). Treg cell inhibition and co-stimulation of effector T cells were shown to be necessary for tumor growth inhibition of OX40 agonists (Piconese et al., 2008, J Exp Med. 205:825-39). Many strategies and technologies have been explored to enhance the anti-tumor effect of OX40 agonist therapy through combinations with vaccines, chemotherapy, radiotherapy, and immunotherapy (Jensen et al., 2010, Semin Oncol. 37:524-32; Melero et al., 2013, Clin Cancer Res. 19:997-1008).